An instrument for terrestrial observation is known comprising:                a constellation of satellites, named the spatial layer for acquisition of images, each satellite being equipped with at least one optical device for acquisition of images of portions of the terrestrial globe and including at least one module for transmission of the acquired images to a terrestrial layer,        a plurality of receiving stations distributed over the surface of the globe, named the terrestrial layer, each receiving station including at least one module for receiving images stemming from the spatial layer.        
The SPOT constellation of satellites constitutes such an instrument. This constellation comprises a plurality of satellites placed in a heliosynchronous, circular, polar orbit and in phase in relation to the Earth. Each satellite has a cycle of 26 days. Each satellite includes optical instruments, data recorders and systems for transmission of the images to receiving stations on the ground. The optical instruments are suitable to ensure parameterisable oblique aims, so that the same region can be observed several times in the course of the cycle of 26 days. The data recorders enable the images to be stored on board if the satellite is not in the line of sight of a ground station. When a satellite is in the line of sight of a ground station, it transmits, by means of a module for transmission of images, the images stored in the on board recorders to the ground station. Each ground station, in turn, then transmits the images to a central server for processing and storage of the images. The same occurs with many other known instruments, for example the Landsat constellation.
One drawback of such an instrument lies in the cost of manufacture, installation, management and maintenance of the constellation of satellites, of the ground stations and of the central server. In particular, such an instrument necessitates the installation and management on the ground of a plurality of ground stations operated in autonomous manner in relation to one another and suitable to send the received images to a central server for backup of the images in a view of their distribution to users.
Furthermore, such an instrument comes up against a major practical difficulty in the case where it is sought to obtain a coverage of the whole of the terrestrial globe by images that exhibit a spatial resolution of the order of one meter and a frequency of global coverage of the order of one day. Such a coverage compels management by the central server of around 1500 terabytes of data per day, that is to say, 170 gigabytes per second, largely exceeding the capacities of satellite transmission and ground processing that are attainable with the technologies and architectures of the servers currently available.
The invention aims to overcome these drawbacks and to provide an instrument for acquisition and downloading of satellite data that enables the acquisition and downloading of a large amount of data.
The invention also aims to provide such an instrument that does not necessitate a central server for processing and backup of the data.